1. Field of the Invention
The present invention relates to a controller for a welding machine of the type including a welding transformer which has a primary winding circuit and a secondary winding circuit, the secondary winding circuit including a pair of welding electrodes between which a workpiece to be welded is held, supply of welding current to the primary winding circuit causing resistance heating of the workpiece.
2. Description of the Prior Art
In the welding machine of this type, too small welding current cannot provide satisfactory welding, while too large welding current will cause flash or spattering which will disadvantageously degrade appearance of the welded portion. For this purpose, the welding current must be properly regulated. Furthermore, repeated welding operations cause deformation of the welding electrodes, which results in corresponding change of a proper value for the welding current. Therefore, the controller for the welding machine is required to have a function of determining a proper value for the welding current in such a condition and regulating the welding current to the determined value.
In order to perform the above control, measurement of the interelectrode resistance is effective. There has been proposed a technique of regulating the welding current in such a way as to cause predetermined change of the interelectrode resistance. An example of such a technique is described in Japanese Laid-Open Patent Publication No. 62-18273.
In the above prior art, the primary winding current and voltage across the electrodes are measured to derive the interelectrode resistance. It is, then, necessary in the prior art to add a voltmeter between the electrodes. Such addition of the voltmeter between the electrode, however, requires specific machining of a gun holding the electrodes so as to place lead wires. Furthermore, in many cases, such a gun is disposed at an inner portion of a welding jig, resulting in difficulty in maintenance. There often occurs a problem of breaking of the lead wires caused by movement of the gun or a workpiece, or flash generated during welding operation. These problems have limited application of the method of controlling a welding machine through interelectrode resistance to actual production lines.
In such a prior art technique of control based upon the interelectrode resistance, a target value or curve is previously set, and the welding current is controlled so as to be equal to the target. Such control of the welding current to be equal to the target will, however, sometimes fail to provide satisfactory welding due to variations of thickness of the workpiece, thickness of a plated layer of the workpiece or the like.